Torque converters having variable pitch stators are desirable for reducing the tendency of the vehicle to creep when the vehicle is idling and the transmission is in a drive gear range. Reduction of creep torque offers several benefits. Fuel economy can be increased. The engine load at idle can be decreased, thereby reducing engine vibration perceived by the driver. And further, for vehicles requiring elevated idle speeds to satisfy high electrical power requirements at idle, idle speed can be increased while maintaining the same creep power.
Variable pitch stators are well known in the art of torque converter design. In most such stators, each stator blade is mounted for rotation about a blade axis passing through the blade. These blade axes typically intersect the torque converter axis.
Both active and passive means are used to provide blade rotation. U.S. Pat. No. 3,398,532, issued to Egbert et al. on Aug. 27, 1968, shows an example of active rotation with the blades being rotated to one of three positions by axial motion of a fluid driven piston. U.S. Pat. No. 2,015,300, issued to C. Dell et al. on Sep. 24, 1935, shows a passive system, wherein the blades rotate in response to fluid force against the blades.
The angle of the stator blades with respect to the torque converter axis changes as the blades are rotated about their respective blade axes. Generally, when the stator blades are rotated to a high inlet angle or closed position, the torque converter capacity for multiplying input torque at a given engine speed is substantially reduced. The blades are typically in the closed position when the vehicle engine is idling. This has the effect of reducing the torque transmitted to the vehicle wheels, thereby substantially reducing the tendency of the vehicle to creep.
However, the disruption of the fluid flow within the torque converter caused by the blades being in the closed position results in vibrations being generated within the torque converter. These vibrations are transmitted through the vehicle to the driver as audible noise and unpleasant vibrations. This condition typically occurs at idle. Also, variable pitch stators have been contributors to harshness of torque converter clutch engaging and disengaging.